Astronomers have made a 42-day movie showing
unprecedented detail of the inner workings of a
strange star system that has puzzled scientists for more
than two decades. Their work is providing new insights
that are changing scientists' understanding of the enigmatic
stellar pairs known as microquasars.

"This once-a-day series of exquisitely-detailed images
is the best look anyone has ever had at a microquasar,
and already has made us change our thinking about how
these things work," said Amy Mioduszewski, of the
National Radio Astronomy Observatory (NRAO), in Socorro,
New Mexico.

The astronomers used the National Science Foundation's
Very Long Baseline Array (VLBA), a system of radio
telescopes stretching from Hawaii to the Caribbean,
to follow daily changes in a binary-star system called
SS 433, some 15,000 light-years from Earth in the
constellation Aquila. Mioduszewski worked with Michael
Rupen, Greg Taylor and Craig Walker, all of NRAO. They
reported their findings to the
American Astronomical Society's meeting in
Atlanta, Georgia.

SS 433 consists of a
neutron star or
black hole orbited
by a "normal" companion star. The powerful gravity of
the neutron star or black hole is drawing material
from the stellar wind of its companion into an
accretion disk of material tightly circling the dense, central object
prior to being pulled onto that object. This disk propels
jets of subatomic particles outward from its poles. In
SS 433, the particles in the jets move at 26 percent of the
speed of light; in other microquasars, the jet material moves
at 90-95 percent of light speed. The disk in SS 433 wobbles
like a child's top, causing its jets to move in a circle
every 164 days.

By imaging SS 433 daily, the astronomers were able to
trace individual ejections of material in these jets as
they moved outward from the center. In addition, they
could track the jets' precession, the movement caused
by the disk's wobble.

In other microquasars, blobs of material shot from
the core become fainter, as seen with radio telescopes,
as they move outward. However, in SS 433, blobs routinely
brighten at specific distances from the core. From earlier
studies, researchers had concluded that such brightening
always occurs at one specific distance. The VLBA
movie shows, instead, that there are multiple brightening
regions and not all blobs brighten at all the regions.

"We think the ejected material brightens because it's
slamming into something," Rupen said. "However, whatever
it's hitting has to be replenished somehow so that the
brightening can occur again when the jet sweeps through that
area the next time," he added.

The VLBA movie revealed vital new information about another
part of SS 433 -- material moving outward from the core,
but not part of the superfast jets. This material moves
outward in a direction not quite perpendicular to the
direction of the jets. Discovered with the VLBA in 2000,
this material had been seen only in one-time snapshots
before, but the movie shows its the steady evolution of
its movement for the first time.

That motion was the key to a possible answer to two
riddles -- the source of the slower-moving material
itself and the source of whatever the jet blobs are
hitting when they brighten.

"What seems most plausible to us is that the accretion
disk is putting out a broad wind," Rupen explained.

That broad wind from the disk hits a denser wind coming
from the "normal" companion star to generate the radio
waves seen coming from the nonjet region. The same
disk-generated wind could be the source of the material
that replenishes the regions where the jet blobs brighten,
the researchers say.

"The motion we measure for this slower-moving material is
fast enough -- about 10,000 kilometers per second -- to
put new material in a brightening region before the jet
circles around to that spot again," Mioduszewski said.

Because accretion disks like that around the dense central
star of SS 433 are known to be unstable, any wind put out
by such a disk could vary, putting out symmetric chunks
in opposite directions. This, the scientists think, may
explain why the jet brightening regions don't always
get replenished with the material needed to cause brightening.

"We still have more questions than answers about this
microquasar, but the VLBA movie shows us that following
the system on a daily basis with such greatly-detailed
images is the most powerful tool available so far to
understand these phenomena," Rupen said.

The astronomers now hope to follow SS 433 with the VLBA
for an entire, 164-day cycle of the jet wobble. At the
same time, they would like to observe the object with
visible-light telescopes, then follow up with larger-
scale images using the NSF's
Very Large Array (VLA) radio
telescope. The VLA images would trace blob motions in
the jets beyond the distances traced with the VLBA.

SS 433 and Microquasars

SS 433 was first noted in the 1960s by astronomers Bruce
Stephenson and Nicholas Sanduleak, who included it in a
catalog they published of stars with unusual features in
their spectra. As the 433rd object in Stephenson and
Sanduleak's catalog, it became known as SS 433.

In 1978, David Clark and Paul Murdin identified SS 433
as the visible-light counterpart of a small object
that had been found to be emitting both radio
waves and X-rays. The small object also sat within
a large supernova remnant called W50. Clark and Murdin,
using the Anglo-Australian Telescope in Australia,
also produced a spectrum of SS 433 that showed strange
features. In addition, the object not only varied in
its brightness, but features within the spectrum
changed.

By 1979, further research, including work by Bruce
Margon and George Abell, had shown that SS 433 was
producing jets of material moving in opposite directions.
The strange stellar system received a wealth of media
coverage, dubbed "the star that is both coming and
going" in one story. A 1981 Sky & Telescope article
was entitled, "SS433 -- Enigma of the Century."

The late Robert Hjellming of NRAO spearheaded studies
of motions within the radio-emitting jets of SS 433 in
the early 1980s.

Because microquasars in our own Milky Way Galaxy are
thought to produce their high-speed jets of material
through processes similar to those that produce jets
from the cores of galaxies, the nearby microquasars
serve as a convenient "laboratory" for studying the
physics of jets. The microquasars are closer and
show changes more quickly than their larger cousins.

The Very Long Baseline Array

The VLBA is a system of ten radio-telescope antennas, each
with a dish 25 meters (82 feet) in diameter and weighing
240 tons. From Mauna Kea on the Big Island of Hawaii to
St. Croix in the U.S. Virgin Islands, the VLBA spans
more than 5,000 miles, providing astronomers with the
sharpest vision of any telescope on Earth or in space.
Dedicated in 1993, the VLBA has an ability to see fine
detail equivalent to being able to stand in New York and
read a newspaper in Los Angeles.

The VLBA's scientific achievements include making the
most accurate distance measurement ever made of an object
beyond the Milky Way Galaxy; the first mapping of the
magnetic field of a star other than the Sun; movies of
motions in powerful cosmic jets and of distant supernova
explosions; the first measurement of the propagation speed
of gravity; and long-term measurements that have improved
the reference frame used to map the Universe and detect
tectonic motions of Earth's continents.